peg poetry version to compat with 3.7

Author: bckohan

.github/workflows/test.yml

@@ -32,6 +32,7 @@ jobs:
       - name: Install Poetry
         uses: snok/install-poetry@v1
         with:
+          version: 1.5.1
           virtualenvs-create: true
           virtualenvs-in-project: true
       - name: Install Dependencies